// https://www.nowcoder.com/practice/b58434e200a648c589ca2063f1faf58c

#include <iostream>
#include <vector>
using namespace std;

struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) : val(x), next(nullptr) {}
};

class Solution {
private:
    ListNode* reverse(ListNode* head) {
        ListNode* curr = head;
        ListNode* prev = nullptr;
        while (curr) {
            ListNode* next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param head ListNode类
     * @param m int整型
     * @param n int整型
     * @return ListNode类
     */
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        ListNode* preHead = new ListNode(0);
        preHead->next = head;
        ListNode* preStart = preHead;
        ListNode* end = preHead;
        for (int i=1; i<m; i++) {
            preStart = preStart->next;
        }
        for (int i=1; i<=n; i++) {
            end = end->next;
        }

        ListNode* start = preStart->next;
        ListNode* c = nullptr;
        if (end != nullptr) {
            c = end->next;
            end->next = nullptr;
        }
        ListNode* t = reverse(start);

        preStart->next = t;

        ListNode* p = preHead;
        while (p->next != nullptr) {
            p = p->next;
        }
        p->next = c;

        p = preHead->next;
        delete preHead;
        return p;
    }
};